The interaction of high-speed turbulence with flames: Turbulent flame speed

Journal Article · Tue Feb 15 04:00:00 EST 2011 · Combustion and Flame

DOI:https://doi.org/10.1016/J.COMBUSTFLAME.2010.09.002· OSTI ID:21401359

Poludnenko, A Y; Oran, E S ^[1]

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1. Laboratory for Computational Physics and Fluid Dynamics, Naval Research Laboratory, Washington, DC 20375 (United States)

Direct numerical simulations of the interaction of a premixed flame with driven, subsonic, homogeneous, isotropic, Kolmogorov-type turbulence in an unconfined system are used to study the mechanisms determining the turbulent flame speed, S{sub T}, in the thin reaction zone regime. High intensity turbulence is considered with the r.m.s. velocity 35 times the laminar flame speed, S{sub L}, resulting in the Damkoehler number Da=0.05. The simulations were performed with Athena-RFX, a massively parallel, fully compressible, high-order, dimensionally unsplit, reactive-flow code. A simplified reaction-diffusion model, based on the one-step Arrhenius kinetics, represents a stoichiometric H{sub 2}-air mixture under the assumption of the Lewis number Le=1. Global properties and the internal structure of the flame were analyzed in an earlier paper, which showed that this system represents turbulent combustion in the thin reaction zone regime. This paper demonstrates that: (1) The flame brush has a complex internal structure, in which the isosurfaces of higher fuel mass fractions are folded on progressively smaller scales. (2) Global properties of the turbulent flame are best represented by the structure of the region of peak reaction rate, which defines the flame surface. (3) In the thin reaction zone regime, S{sub T} is predominantly determined by the increase of the flame surface area, A{sub T}, caused by turbulence. (4) The observed increase of S{sub T} relative to S{sub L} exceeds the corresponding increase of A{sub T} relative to the surface area of the planar laminar flame, on average, by {approx}14%, varying from only a few percent to as high as {approx}30%. (5) This exaggerated response is the result of tight flame packing by turbulence, which causes frequent flame collisions and formation of regions of high flame curvature >or similar 1/{delta}{sub L}, or ''cusps,'' where {delta}{sub L} is the thermal width of the laminar flame. (6) The local flame speed in the cusps substantially exceeds its laminar value, which results in a disproportionately large contribution of cusps to S{sub T} compared with the flame surface area in them. (7) A criterion is established for transition to the regime significantly influenced by cusp formation. In particular, at Karlovitz numbers Ka >or similar 20, flame collisions provide an important mechanism controlling S{sub T}, in addition to the increase of A{sub T} by large-scale motions and the potential enhancement of diffusive transport by small-scale turbulence. (author)

• OSTI ID: 21401359
• Journal Information: Combustion and Flame, Journal Name: Combustion and Flame Journal Issue: 2 Vol. 158; ISSN CBFMAO; ISSN 0010-2180
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
AIR
COMBUSTION
COMBUSTION KINETICS
COMPUTERIZED SIMULATION
CUSPED GEOMETRIES
DIFFUSION
FLAME PROPAGATION
Flamelets
HYDROGEN
LAMINAR FLAMES
LEWIS NUMBER
MIXTURES
Premixed combustion
STOICHIOMETRY
SURFACE AREA
SURFACES
TURBULENCE
VELOCITY
ZONES